A standard power planer has a drum that is rotated at high speed about a horizontal axis and that carries a plurality of blades that themselves define a cutting orbit when the drum rotates. The apparatus further has a table that the drum projects up from so that the workpiece to be planed can be slid across the table, with the rotating drum cutting into and smoothing the workpiece. The part of the table downstream in the workpiece-travel direction from the blade is set to be level and tangent with the orbit of the drum, so that the planed surface of the workpiece will slide smoothly along it, whereas the upstream part of the table is set lower by a distance equal to the amount of material that is to be removed from the bottom side of the workpiece. Such an apparatus can effectively and rapidly smooth one side of the workpiece.
A two-sided planer is also known (See pp. 590ff of Holz Lexicon by Ewald Konig, published by DRW Verlag in Stuttgart, 1972.) which has two vertically spaced drums, the upper one rotating about an axis parallel to that of the lower one and being spaced downstream therefrom. The vertical position of the downstream drum can be adjusted to set the amount this drum planes off the upper side of the workpiece. A workpiece passed once through such a machine has its upper and lower sides both planed, establishing a set workpiece thickness. Such a machine is normally used to make finish-grade boards, and can even be set up with a second pair of cutter drums perpendicular to the first to plane and square the upright sides of the board also.
This type of multiside planer is extremely tedious to set up to produce a given board thickness. Normally the amount that is taken off the bottom side of the workpiece is always the same, and the upper cutter is set to take off the balance. Obviously this means that more is cut off one side of the board than the other. With such unequal planing the feed speed must be set in accordance with the amount being taken off on the side with the deeper cut, as travel speed must drop when a thick cut is being made if waviness is to be avoided and the tool to be preserved.
In general the operation of the known multiside planers is unsatisfactory in that feed speed must be held low for the reasons described above, the material of the workpiece is wasted, setup is very complex, and it is normally impossible to adjust thickness during a run. In fact any adjustment normally entails considerable down time for the machine.